Lord-Lucan points well taken!!
Normally I would recommend for folks to disable the debug UART during deployments. That way there’s no extra power draw from those pins.
Additionally, you could check the RX pin to see if it’s high. It will go low when the CP2102 is no longer powered. I haven’t tested it but that was my solution when I thought about this problem previously. If there were extra GPIOs I would have added a dedicated detection pin. Maybe they’ll make a newer nRF91 with more pins in the future 😄